"Automobiles, wagons, boats and other vehicles used in the unlawful
transportation of intoxicating liquors are subject to seizure and
forfeiture. No search of any vehicle may be made unless the officer
has reasonable grounds for believing that such vehicle is being used
for the violation of the prohibition laws. There can be no search
without legal probable cause for such search. . . .

"Chief Justice Taft, in the Carroll case, said: 'It would be intolerable
and unreasonable if a prohibition agent were...